A wealthy, quiet beach city was overrun on the Fourth of July, with hundreds arrested — bad news for residents and a sign that "teen takeover" mobs are spreading to unexpected places.

• Newport Beach, one of California's richest and safest towns, saw 400 arrests as crowds fought each other, looted stores, and threw fireworks into the crowd.
• Fights started early in the evening; police were overwhelmed and had to call in officers on horseback for backup.
• One officer was knocked down when a firework exploded against her chest; others were injured.
• The trouble is blamed on outsiders who organize "takeover" mobs on Snapchat and Instagram and show up to trash events — not local residents.
• Nearby Huntington Beach avoided the same fate in the past when locals and biker groups turned out to block troublemakers.

Outlook: Expect pressure on city leaders and calls for tougher punishments, as these social-media-driven youth mobs keep targeting new cities.